1. Field of the Invention
The present invention relates to a method for inputting and outputting signals representative of full color containing half-tone, which are used for a color printer, color copying machine, or the like, and an apparatus for executing the method. More particularly, the invention relates to a method for transforming color signals in order to reproduce a color faithfully, and an apparatus for executing the method.
2. Description of the Related Art
In the fields of color printing, color television, color copying machine, and the like, there are many proposals to transform color signals. In a typical example of those proposals, an input color space, e.g., a BGR coordinate, is directly transformed into an output color space, e.g., a YMC (K) coordinate by using a table memory. When three color signals in a BGR coordinate, for example, are transformed into digital signals at resolutions of the necessary gray levels, a great amount of table data is needed. To store the data, a table memory with a large memory capacity must be provided. Such a memory is very expensive.
For example, in a case where each of the input colors B, G, and R are expressed by 8 bits, and the output colors Y, M, C, and K are also expressed by 8 bits, the required memory capacity of the table memory is 2.sup.24 .times.4 bytes. It is impractical to use such a big table memory.
Many interpolation basis methods have been studied for reducing the necessary memory capacity in transforming color signals by using the table memory. In these methods, a color correction memory addressed with the higher bits of the input signals is used for reducing the necessary memory capacity. The coarsened data is corrected by using an interpolation circuit using the lower bits. The interpolation basis method is disclosed, for example, in Published Unexamined Patent Application No. Hei 2187374 and Examined Japanese Patent Application No. Sho. 58-16180. But, this interpolation is discontinuous at the boundary between the adjacent interpolation regions.
To solve the above problems, the inventor of the present Patent Application proposes in a co-pending U.S. Patent Application Serial No. (unknown) filed on the same day as the present Patent Application was filed.
The interpolation method of the co-pending application will be described with reference to FIG. 9 and an equation (1). ##EQU1##
where X'(x, y, z): a value of one output for an input (x, y, z)
x.sub.h, y.sub.h, z.sub.h : higher bits of the input signals
x.sub.1, y.sub.1, z.sub.1 : lower bits of the input signals
X'(x.sub.h, y.sub.h, z.sub.h): basic data obtained from the combination of the higher bits
a.sub.x (x.sub.h, y.sub.h, z.sub.h, d.sub.x (x.sub.1, y.sub.1, z.sub.1)), a.sub.y (x.sub.h, y.sub.h, z.sub.h,
d.sub.y (x.sub.1, y.sub.1, z.sub.1)), a.sub.z (x.sub.h, z.sub.h, d.sub.jz (x.sub.1, z.sub.1)): interpolation sensitivity signals
d.sub.x (x.sub.1, y.sub.1, z.sub.1), d.sub.y (x.sub.1, y.sub.1, z.sub.1), d.sub.z (x.sub.1, y.sub.1, z.sub.1):
interpolation region select signals obtained from the combination of the lower bits
The operation of the color signal transforming apparatus shown in FIG. 9 will be described briefly.
Three input signals for instance, L*, a*, and b* representing colors are each divided into the higher bits and the lower bits. The combination of the higher bits is input as an address signal to a color correction memory 1 for basic data, which then outputs basic data corresponding to X'(x.sub.h, y.sub.h, z.sub.h).
The combination of the lower bits is input as an address signal to the interpolation region select memory 7, which outputs the interpolation region select signals corresponding to d.sub.x (x.sub.1, y.sub.1, z.sub.1), d.sub.y (x.sub.1, y.sub.1, z.sub.1, ), d.sub.z (x.sub.1, y.sub.1, z.sub.1).
The combination of the higher bits of the input signals corresponding to d.sub.x (x.sub.1, y.sub.1, z.sub.1), d.sub.y (x.sub.1, y.sub.1, z.sub.1), d.sub.z (x.sub.1, y.sub.1, z.sub.1).
The combination of the higher bits of the input signals and some of the interpolation region select signals are input to interpolation sensitivity signal output memories 5.sub.1, 5.sub.2, and 5.sub.3, which then output interpolation sensitivity signals corresponding to a.sub.x (x.sub.h, y.sub.h, z.sub.h, d.sub.x (x.sub.1, y.sub.1, z.sub.1)), a.sub.y (x.sub.h, y.sub.h, z.sub.h, d.sub.y (x.sub.1, y.sub.1, z.sub.1)), and a.sub.z (x.sub.h, y.sub.h, z.sub.h, d.sub.z (x.sub.1, y.sub.1, z.sub.1)).
The interpolation sensitivity signals are respectively input to interpolation multipliers 6.sub.1 to 6.sub.3 where these are multiplied by the lower bits of the input signals, respectively. The multipliers produce the resultant products in the form of plural interpolation data.
Finally, interpolation adders 3.sub.1 to 3.sub.3 respectively add the basic data and the plural interpolation data. The resultant sums are output as interpolated values corresponding to X'(x, y, z) in the equation (1).
The interpolation method has the following advantageous features:
(1) Complicated address calculation is not required when the memory is accessed. High speed processing is possible with a simple circuit arrangement.
(2) The regular arrangement in the color correction memory is not essential. The technique of the invention is compatible with the technique, which removes the memory portion out of the color reproduction regions by arranging irregularly the data that are regularly arranged in the memory (Published Unexamined Japanese Patent Application Nos. Hei. 2-73779 and 2-187374).
(3) Interpolation continuity at the boundary region is secured.
The interpolation method as mentioned above is incapable of handling such a case where a quantity of black K is intended to be variable according to the input characteristics, not fixedly set for the whole color space, when the output signals are color signals of Y, M, C, and K.
The problem will be described specifically.
When four color signals of Y, M, C, and K are output to an output device, such as a xerography machine, in view of the graininess on an image, a picture area contains black K preferably at the smallest possible quantity. In view of the miss registration, character portion contains black K preferably at the largest possible quantity.
In an extreme case, the picture area is output with three colors Y, M and C, while the character portion is output with any of the combinations of two colors and black K, such as the combination of Y, M and K, the combination of M, C, and K, and the combination of C, Y, and K. Practically, it has been desired to realize the output color combinations moderate but closer to those of the extreme case.
To secure a continuity of image at the boundary regions when the percentage of black K contained is changed, it is preferable to increase the number of the moderate color combinations as large as possible.
The attempt to realize this by modifying the interpolation method proposed in the co-pending application as stated above may be categorized into the following two:
(1) The basic data color correction memory and the interpolation sensitivity signal output memory are constructed with reprogrammable memories. When another percentage of the contained black K is required, the content of the memory is replaced with the percentage.
(2) As shown in FIG. 10, a K-control input is additionally used for the addresses of the color correction memory and the interpolation sensitivity signal output memories, as shown in FIG. 10.
The above two attempts have the following problems, however. In the case of (1), the memory content replacement takes much time. When it is necessary to change the percentage of the contained black K in one image, the replacing operation time will be too long time for printing. In the case of (2), when the number of bits of the K control input signal is increased, that is, the number of percentages of the contained K, is increased, the memory capacities of the color correction memory and the interpolation sensitivity signal output memories must be increased by the amount corresponding to the increased number. This leads to increase of the cost to manufacture.